Proteins are useful in a variety of diagnostic, pharmacologic, agricultural, nutritional, and research applications. Given the high cost of producing proteins, especially therapeutic proteins, even small increases in the efficiency of production or in the function and stability of a protein can be valuable. The function and stability, and hence the utility, of a protein can be affected by the posttranslational addition of sugar residues to the protein to form a glycoprotein. For example, the addition of terminal sialic acid residues to polysaccharides attached to a glycoprotein generally increases the protein's lifetime in the bloodstream and can, in particular cases, also affect solubility, thermal stability, resistance to protease attack, antigenicity, and specific activity of some glycoproteins. See e.g. Gu and Wang (1998), Biotechnol. and Bioeng. 58(6): 642-48; Morell et al. (1968), J. Biol. Chem. 243(1): 155-59. It is therefore desirable to increase the sialic acid content of a glycoprotein, especially a glycoprotein to be used for pharmacologic applications.